1. Field of the Invention
The present invention relates to a thermal head and a manufacturing method therefor, and a printer including the thermal head.
2. Description of the Related Art
There has been conventionally known a thermal head used in a thermal printer to effect printing onto a thermosensitive recording medium by selectively driving a plurality of heating elements based on printing data (see, for example, JP 2007-83532 A).
As a method for achieving a reduction in power consumption by improving thermal efficiency of a heating resistor in a thermal head, there has been known a method in which a hollow portion is formed in a region opposing the heating resistor. By allowing the hollow portion to function as a heat insulating layer having a low thermal conductivity, and reducing an amount of heat propagated and dissipated from the heating resistor to a support substrate, efficiency of energy used for printing may be improved.
Such a thermal head having a hollow portion is formed by providing a silicon substrate (lower plate substrate) with a concave portion by etching or laser processing, bonding a glass thin plate (upper plate substrate) serving as a heat accumulating layer onto the silicon substrate, and then processing the upper plate substrate to a desired thickness by polishing. Then, a heating resistor and wiring for power supply are formed on the upper plate substrate thus polished, and covered with a protective film, whereby the thermal head is formed.
For a conventional thermal head which does not have a hollow portion, methods are known which aim at preventing the separation of a protective film and a heating resistor from an upper plate substrate. One method involves adjusting the surface roughness Ra of the upper plate substrate to be 0.01 to 0.2 μm (10 to 200 nm) to improve the adhesion of the heating resistor (see, e.g., JP 60-210469 A). Another method involves adjusting the surface roughness of the upper plate substrate to be not less than 5 nm to improve the adhesion of the protective film (see, e.g., JP 06-340103 A).
However, in the thermal head having the hollow portion, the upper plate substrate is a glass plate as thin as 10 to 100 μm, and accordingly has a structure extremely weak to a force from above.
In general, it is said that the theoretical strength of glass is higher than that of iron. However, it has been known that, if there is a scratch or a defect in a surface of glass, a stress is concentrated thereon, and the strength of glass is reduced to about 1/10 to 1/100 times the theoretical strength.
In addition, the strength of glass decreases as the scratch is deeper and the number of scratches is larger so that the strength decreases as the surface roughness is larger. Accordingly, when the surface of a glass thin plate as the upper plate substrate is roughened to improve the adhesion of a film, a problem occurs that the resulting thermal head has an extremely low strength with respect to the concentrated stress.